Obesity is a growing health concern. As obesity has increased, the amount of physical activity in the population has decreased. In our sedentary society, low levels of physical activity have a significant impact on weight gain. Whereas the neural, hormonal, molecular, and genetic mechanisms of energy intake are well studied, very little is known about the mechanisms through which energy is expended through physical activity. Non-exercise activity thermogenesis (NEAT) in humans is comprised of the energy expenditure of all physical activity outside of volitional exercise;NEAT contributes significantly to the ability to resist weight gain in the face of overfeeding. We have previously demonstrated that central administration of the neuropeptide orexin A increases NEAT in rats. Moreover, obesity-prone rats are less sensitive to the NEAT-activating effects of orexin in the paraventricular hypothalamic nucleus (PVN) after access to a high-fat diet compared to obesity-resistant rats. Here, we propose to systematically examine the mechanisms through which PVN orexin differentially affects NEAT in obese and lean animals. We hypothesize that the energy expenditure from orexin-induced NEAT induces weight loss. Therefore, we will determine if twice-daily central orexin treatment decreases body weight and increases NEAT in rats. Next, we hypothesize that decreases in orexin-induced NEAT contribute to weight gain in obesity-prone rats. Therefore, in the second study, we will determine how the high-fat diet contributes to the ability of orexin to stimulate NEAT in the obesity-prone, obesity-resistant, and control rats. Third, we will determine how a high-carbohydrate diet alters daily NEAT in obesity-prone, obesity-resistant, and control rats, as well as how the diet affects orexin-induced NEAT. Lastly, we will examine the neural mechanisms of the differential effects of orexin on NEAT in obesity-prone, obesity- resistant, and control rats by comparing orexin synthesis and orexin content in targeted brain regions in these animals. With these studies, we hope to delineate the neural mechanisms of how orexin affects changes in NEAT, as well as to assess how orexin interacts with diet to affect NEAT and obesity. Understanding the mechanisms through which the energy expenditure of physical activity affects NEAT is a critical step in developing effective behavioral and pharmacological strategies to combat obesity.